1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using a laser source and a control method thereof.
2. Description of the Related Art
In an electrophotographic image forming apparatus using a laser source, a uniformly charged photosensitive drum rotates in a predetermined direction (sub-scanning direction), and the laser source emits a laser beam corresponding to an image signal. While scanned right and left (main scanning direction), the laser beam irradiates the photosensitive drum, forming an electrostatic latent image on it. The developing unit supplies toner to the photosensitive drum to visualize the electrostatic latent image on the photosensitive drum into a toner image.
An image forming process using a laser beam will be explained with reference to FIGS. 9 to 12B. FIG. 9 is a view schematically showing the potential relationship between a portion irradiated with a laser beam and an unirradiated portion on the photosensitive drum. FIG. 10 is a view schematically showing toner attachment states at portions irradiated with laser beams of different irradiation pulse widths on the photosensitive drum. FIG. 11 is a view showing toner attachment states at portions irradiated with laser beams of the same light quantity on the photosensitive drum. FIGS. 12A and 12B are graphs schematically showing sensitivity variations of the photosensitive drum in the main scanning direction and sub-scanning direction.
If a laser beam L irradiates the photosensitive drum uniformly charged to a potential VD, the potential of a portion irradiated with the laser beam L drops to a potential VL lower than the potential VD, as shown in FIG. 9. Toner supplied from the developing unit attaches to the portion at the potential VL, forming a toner image.
The laser source is ON/OFF-controlled to change the irradiation pulse width of the laser beam based on an image signal. This control method, called PWM, changes the ratio of the ON and OFF periods of the laser source. For example, the laser source can irradiate the photosensitive drum with a laser beam L1 and a laser beam L2 narrower in pulse width than the laser beam L1, as shown in FIG. 10. The amount of toner attached to a portion irradiated with the laser beam L1 is larger than that attached to a portion irradiated with the laser beam L2. That is, the amount of attached toner changes to express the density of an image transferred onto paper by changing the ratio of the ON and OFF periods of the laser source.
However, even if laser beams L3 of the same light quantity irradiate the photosensitive drum, sensitivity variations between portions on the photosensitive drum result in different potentials VL3 and VL4, as shown in FIG. 11, and the image density may vary. For example, a highly durable amorphous silicon photosensitive drum readily varies in film thickness and film quality in the manufacture. Variations in film thickness and film quality lead to variations in sensitivity to electrification and exposure. When a predetermined quantity of laser beam irradiates the amorphous silicon photosensitive drum, potential variations as shown in FIG. 12A occur in the main scanning direction of the photosensitive drum, and those as shown in FIG. 12B occur in the sub-scanning direction.
To solve these problems, there has been proposed a technique of downloading, from a data server, correction data to correct sensitivity variations unique to each photosensitive body (photosensitive drum), and correcting laser power in accordance with the exposure position on the photosensitive body based on the correction data (see, e.g., Japanese Patent Laid-Open No. 2004-345170).
There has also been proposed a technique of rotating a photosensitive body while scanning a surface electromer in the main scanning direction, spirally measuring sensitivity variations by one turn of the photosensitive body, and correcting the exposure in accordance with the exposure position on the photosensitive body using the measurement result (see, e.g., Japanese Patent Laid-Open No. 2004-258482). To detect the rotational position of the photosensitive body, the photosensitive body comprises a home position member (HP indication means) indicating the home position. The photosensitive body also comprises a home position sensor (HP detection means) which detects the home position member. The home position sensor is positioned to detect the home position member when the home position member reaches a preset reference position (home position). The number of main scanning lines is counted from the detected home position serving as a reference. The rotational position of the photosensitive body, that is, the exposure position on the photosensitive body is calculated from the count value. The calculated exposure position and measured photosensitive body sensitivity are held in correspondence with each other.
When the above-mentioned techniques are applied, the exposure position on the photosensitive drum must be accurately calculated in actual image formation in order to correct the exposure in accordance with the exposure position on the photosensitive body. This is because correction data corresponding to the exposure position must be used.
In actual image formation, however, the rotational speed of the photosensitive drum may temporarily vary due to, for example, impact force acting on the photosensitive drum when paper enters between the photosensitive drum and the intermediate transfer belt. Thus, an exposure position on the photosensitive drum that is calculated based on the number of main scanning lines counted from the home position serving as a reference may not coincide with an exposure position on the photosensitive drum in actual image formation. Correction data which should be used for an actual exposure position may become different from correction data corresponding to an exposure position on the photosensitive drum that is calculated from the home position serving as a reference. It is, therefore, difficult to properly control the exposure in accordance with sensitivity variations of the photosensitive drum.